The present invention relates to a semiconductor device used in a high frequency band, for example.
Recent years, the semiconductor devices used in the high frequency band having such transistors as a gallium arsenide field effect transistor or the like (hereinafter referred to GaAsFET), for example, as its circuit elements is required to take some measures against heat generated by the transistors during operation with progress in high integration of the elements.
Here, a conventional semiconductor device using GaAs FET is explained referring to FIG. 1.
A semiconductor element 21, a GaAs FET, for example, used for power amplification is mounted on a metal base plate 20. A first dielectric plate 22 is provided on an input side (on the left hand side in the figure) of the semiconductor element 21. On the first dielectric plate 22, a circuit pattern 22a composing, for example, an input side matching circuit etc. is formed. A second dielectric plate 23 is provided on the output side (on the right hand side in the figure) of the semiconductor element 21. On the second dielectric plate 23, a circuit pattern 23a composing an output side matching circuit, for example, is provided. A side wall 24 having a rectangular frame shaped in a plan view is formed with a predetermined height on the base plate 20 surrounding the semiconductor element 21, the first dielectric plate 22 and the second dielectric plate 23. The major portion of the side wall 24 is composed of metal, for example, and the rectangular opening at its upper end is sealed with a metal lid 25. A package is composed of the metal base plate 20, side wall 24 and the metal lid 25.
An input side power supply portion 24a including an insulator is provided in one of the sides of the side wall 24 (on the left hand side in the figure), through which an input line conductor 26a penetrates. An input lead pin 27a is connected with the input line conductor 26a outside the package.
An output side power supply portion 24b including an insulator is provided on another side of the side wall 24 (on the right hand side in the figure), through which an output line conductor 26b penetrates. An output lead pin 27b is connected with the output line conductor 26b outside the package.
The circuit pattern 22a on the first dielectric plate 22 is connected with the input line conductor 26a and with the semiconductor element 21 by wires W1 and by wires W2 respectively. The circuit pattern 23a on the second dielectric plate 23 is connected with the semiconductor element 21 and with the output line conductor 26b by wires W3 and by wires W4 respectively.
In the semiconductor device described above, input signals introduced by the input line 26a are amplified by the semiconductor element 21 and is outputted from the output line 26b. 
In the semiconductor device, heat is generated from the semiconductor element 21 during its operation. The heat generated by the semiconductor element 21 is transmitted to the base plate 20 provided right under the semiconductor element 21. The heat generated from the semiconductor element 21 is transmitted from the region right under the semiconductor element 21 toward the base plate 20 spreading with an angle of about 45° and is radiated from an undersurface of the base plate 20.
A semiconductor device described is disclosed in Japanese published patent application H9-153839, in which semiconductor elements and other circuit elements are enclosed in a package consisting of a side wall provided on a base plate 20 having an opening at its upper end and a lid for covering the opening formed by the side wall.
In the conventional semiconductor device, heat generated in the semiconductor element 21 is radiated outside through the base plate 20 for mounting the semiconductor element 21. Here, heat dissipation property of the base plate 20 is determined by a product of an area of such a heat source as the semiconductor element 21 and heat resistance of the base plate 20. In the conventional semiconductor device, however, efficient dissipation of the heat could not be expected, because the semiconductor element 21 is small and thus the area of the heat source is small.
Further, the base plate 20 is required to have electrical conductivity as well as the heat dissipation property, because the base plate 20 functions as a ground electrode. Therefore, single layer copper or a multilayer copper structure, in which molybdenum or tungsten layer is interposed between copper layers for mechanical reinforcement of the plate, is used as the base plate 20.
Copper has high electricity conductivity and has high heat conductivity among various metal materials. However, the heat conductivity is around 400 W/m·K and sufficient heat dissipation efficiency cannot be obtained, when the semiconductor device is of high integration and heat generation from the semiconductor element 21 is high. Further, heat dissipation of the base plate 20 having the multilayer structure is degraded compared to that of the single layer copper, because the heat conductivity of the molybdenum or tungsten is lower than that of copper.